Engines may use boosting devices, such as turbochargers, to increase engine power density. However, engine knock may occur due to increased combustion temperatures. Knock is especially problematic under boosted conditions due to high charge temperatures. The inventors herein have recognized that utilizing an engine system with a split exhaust system, where a first exhaust manifold routes exhaust gas recirculation (EGR) to an intake of the engine, upstream of a compressor of the turbocharger, and where a second exhaust manifold routes exhaust to a turbine of the turbocharger in an exhaust of the engine, may decrease knock and increase engine efficiency. In such an engine system, each cylinder may include two intake valves and two exhaust valves, where a first set of cylinder exhaust valves (e.g., scavenge exhaust valves) exclusively coupled to the first exhaust manifold may be operated at a different timing than a second set of cylinder exhaust valves (e.g., blowdown exhaust valves) exclusively coupled to the second exhaust manifold, thereby isolating a scavenging portion and blowdown portion of exhaust gases. The timing of the first set of cylinder exhaust valves may also be coordinated with a timing of cylinder intake valves to create a positive valve overlap period where fresh intake air (or a mixture of fresh intake air and EGR), referred to as blowthrough, may flow through the cylinders and back to the intake, upstream of the compressor, via an EGR passage coupled to the first exhaust manifold. Blowthrough air may remove residual exhaust gases from within the cylinders (referred to as scavenging). The inventors herein have recognized that by flowing a first portion of the exhaust gas (e.g., higher pressure exhaust) through the turbine and a higher pressure exhaust passage and flowing a second portion of the exhaust gas (e.g., lower pressure exhaust) and blowthrough air to the compressor inlet, combustion temperatures can be reduced while improving the turbine's work efficiency and engine torque.
However, the inventors herein have recognized potential issues with such systems. As one example, at low intake manifold pressure (MAP) conditions, the first set of cylinder exhaust valves (e.g., scavenge exhaust valves) may be disabled to reduce reverse flow through the EGR passage and into the cylinders. However, under this operation during a cold start, cold start emissions may be higher than desired and engine warm-up may not be adequate.
In one example, the issues described above may be addressed by a method comprising: during a cold start, adjusting a position of a first valve disposed in an exhaust gas recirculation (EGR) passage based on an engine operating condition, the EGR passage coupled between a first exhaust manifold coupled to a first set of exhaust valves and an intake passage, upstream of a compressor, while flowing a portion of exhaust gases to an exhaust passage including a turbine via a second set of exhaust valves. As one example, in response to the first set of exhaust valves (e.g., scavenge exhaust valves) being activated at the start of engine cranking, the first valve may be opened at the start of engine cranking to recirculate any residual hydrocarbons to the compressor inlet to reduce emissions and then, following firing of a first cylinder, the first valve may be modulated to control EGR to a desired level. Further, an activation state of the first set of exhaust valves may be switched in response to an oil pressure reaching a threshold pressure for adjusting the valve activation state. As another example, in response to the first set of exhaust valves being deactivated at the start of engine cranking and after reactivating the first set of exhaust valves, the first valve may be modulated to control EGR to a desired level. In this way, recirculating flow via the first valve may expedite warming up of the incoming air charge and reburn hydrocarbons exiting the cylinder in the recirculation flow, thereby decreasing engine emissions. As a result of controlling the first valve and/or the first set of exhaust valves in response to an engine operating condition during a cold start, cold start emissions may be decreased while also aiding in engine warmup.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.